1. Field of the Invention
The present invention relates to gradation conversion of an image.
2. Description of the Related Art
Gradation conversion is a process in which a pixel value band in an image is highlighted as compared with the other bands. Gradation conversion is used to match the gradation of an image with the gradation during recording and the gradation of a display device, and to increase the gradation of a region of interest. This processing allows an unnecessary loss of highlight detail in the image or loss of shadow detail in the image to be prevented. Further, an image can be obtained in which the region of interest is easier to view.
On the other hand, shooting methods such as large image shooting or fractionated image shooting exist in which a plurality of partial images is obtained by shooting a shooting region of an object while dividing the shooting region into a plurality of shooting ranges. These methods are used to shoot a large object that does not fit in a single image. An image of the whole object can be obtained by joining the individual partial images obtained by large image shooting. U.S. Pat. No. 7,440,559 discusses techniques for converting the gradation of an image obtained by large image shooting. In U.S. Pat. No. 7,440,559, techniques are discussed in which gradation conversion is performed based on a density histogram of a joined image, or based on one partial image.
However, if a joined image is used as an analysis target, the processing load increases, which makes it more difficult to achieve accurate analysis. On the other hand, if the gradation conversion is performed on a large image based on one partial image, since the other partial images are not taken into consideration, the whole large image may not have an appropriate gradation.